Mycoplasma hominis strain 1620 was isolated from the synovial fluid of a chronically infected septic arthritis patient. We have been studying the variable expression of surface exposed, acylated, integral membrane proteins that may play a role in the persistance of this pathogen in its human host. Three monoclonal antibodies (Mabs) that bind specifically to the variable M. hominis surface antigens have been previously constructed and characterized. The Mabs are being used as probes of M. hominis gene banks in our effort to isolate, clone and characterize a gene or genes that encode the protein portion(s) of the membrane lipoproteins. Additionally, we are analyzing the expression of the Mab-specific surface antigens in generations of clones. Individual colonies vary in relative size from small to medium to large. We have determined that the large colonies express high molecular weight proteins that are absent from the smaller colonies. The antigenic switching that occurs is under study.